Born in Germany and educated at Göttingen, Walter Baade worked at the Hamburg Observatory from 1919 to 1931 and at Mt. Wilson from 1931 to 1958. He and Fritz Zwicky proposed in 1934 that supernovae could produce cosmic rays and neutron stars, and Baade made extensive studies of the Crab Nebula and its central star. During World War II blackouts of the Los Angeles area Baade used the 100-inch Hooker telescope to resolve stars in the central region of the Andromeda Galaxy for the first time. This led to his definition of two stellar populations, to the realization that there were two kinds of Cepheid variable stars, and from there to a doubling of the assumed scale of the universe. Baade and Rudolph Minkowski identified and took spectrograms of optical counterparts of many of the first-discovered radio sources, including Cygnus A and Cassiopeia A. Baade discovered the asteroids Hidalgo and Icarus.